How about "My sister and I WhatsApp each other every day"?
You may use the term WhatsApp as an informal verb to mean you communicate using the software application called WhatsApp. In pretty much the same manner, you may turn the term Viber (another software application you exchange instant messages or talk through) into yet an other informal verb. It seems to me that, as a verb, email as in "She emailed me out of the blue a few days ago" may have preceded the two aforementioned verbs.
That said, that's sad.